How does current flow in a copper wire? If it passes through the transfer of electrons, then why does a copper wire still exist, even after losing electrons which are the fundamental particles that build the copper atom?
Valence electrons in a metal are delocalized, which means they don't know to what atom they belong. The cores are bound in a crystal lattice and the outer electrons flow around it like a gas, similar to this. When you attach a voltage to both ends of such a material, the electrons bounce into each other and push each other a tiny bit down the wire. Electrons are flowing in a circle, they are not used up in a circuit. Rather, the power supply pushes them around continuously. In a regular metal conductor, they slow down due to the resistance of the copper. In a superconductor, they don't slow down and so you are able to generate a loop of current that keeps flowing indefinitely.
Apart from that, the fact that a copper atom is a copper atom depends on the number of protons in the nucleus, not the number of electrons. That stays the same even if you were to strip it of all its electrons.
No electrons are being lost; for a current to flow there needs to be a circuit, ie some kind of loop. Also, when current flows, it is not necessarily the electrons themselves that are moving as fast as the current, but rather the signal to move. The copper atom also consists of protons and neutrons in its nucleus, as well as most of its electrons which stay put, locked to the atom itself while only the 'outer' ones move around the metallic crystal.
Only a few electrons in the outer shell of a copper atom are ever displaced (removed) due to charge forces. The rest all stay in place. So even if copper atoms are mediating a current, the integrity of the atoms themselves is never in doubt.